The present invention relates to methods of operating and control systems for refrigeration systems and, more particularly, to methods of operating and control systems for controlling recycle starts of a compressor in a refrigeration system.
Generally, refrigeration systems include an evaporator or cooler, a compressor, and a condenser. Usually, a heat transfer fluid is circulated through tubing in the evaporator thereby forming a heat transfer coil in the evaporator to transfer heat from the heat transfer fluid flowing through the tubing to refrigerant in the evapbrator. The heat transfer fluid chilled in the tubing in the evaporator is normally water which is circulated to a remote location to satisfy a refrigeration load. The refrigerant in the evaporator evaporates as it absorbs heat from the water flowing through the tubing in the evaporator, and the compressor operates to extract this refrigerant vapor from the evaporator, to compress this refrigerant vapor, and to discharge the compressed vapor to the condenser. In the condenser, the refrigerant vapor is condensed and delivered back to the evaporator where the refrigeration cycle begins again.
To maximize operating efficiency, it is desirable to match the amount of work done by the compressor to the work needed to satisfy the refrigeration load placed on the refrigeration system. Commonly, this is done by capacity control means which adjusts the amount of refrigerant vapor flowing through the compressor. The capacity control means may be a device such as guide vanes which are positioned between the compressor and the evaporator and which move between a fully open and a fully closed position in response to the temperature of the chilled water leaving the chilled water coil in the evaporator. When the evaporator chilled water temperature falls, indicating a reduction in refrigeration load on the refrigeration system, the guide vanes move toward their closed position, decreasing the amount of refrigerant vapor flowing through the compressor. This decreases the amount of work that must be done by the compressor thereby decreasing the amount of energy needed to operate the refrigeration system. At the same time, this has the effect of increasing the temperature of the chilled water leaving the evaporator. In contrast, when the temperature of the leaving chilled water rises, indicating an increase in load on the refrigeration system, the guide vanes move toward their fully open position. This increases the amount of vapor flowing through the compressor and the compressor does more work thereby decreasing the temperature of the chilled water leaving the evaporator and allowing the refrigeration system to respond to the increased refrigeration load. In this manner, the compressor operates to maintain the temperature of the chilled water leaving the evaporator at, or within a certain range of, a set point temperature. Under certain operating conditions, such as low load conditions, the refrigeration system may provide excess capacity for satisfying the load placed on the refrigeration system even though the guide vanes are at their fully closed position which corresponds to a minimum operating capacity for the compressor. Under these conditions, it is customary to turn off the refrigeration system compressor to prevent undesirable excess cooling of the water flowing through the heat transfer tubes in the evaporator which, if unchecked, could result in freezing of this water. Then, when a new, increased load on the refrigeration system is detected, the compressor is restarted and the guide vanes are again used to adjust refrigeration system capacity to match the load placed on the refrigeration system. A restart of the refrigeration system compressor under the foregoing conditions is known as a recycle start. Recycle starts are not particularly desirable since they produce wear and tear on the mechanical and electrical systems of the refrigeration system and may reduce the operating life and decrease the reliability of the overall refrigeration system.